In transmitting digital information over comparatively long distances and with high information speed, propagation time soon becomes of the same order of magnitude as or greater than the information time for an individual item of the information in question. The propagation time is limited by the speed of light, which in vacuum is about 300 m/.mu.s. For example, the propagation time for a pair in a cable or in a quad conductor may attain 140 m/.mu.s. Since the information time in communication systems using, for example, pulse code modulated (PCM) transmission to a telephone set, is in the order of magnitude of microseconds, and the line length may often be thousands of meters, the propagation times can easily attain the order of magnitude of tens of microseconds. Some form of synchronization must therefore be resorted to for ensuring reliable information reading.
An usual method is to get the synchronizing information from the information flow itself, e.g. by forming the information such that data, facilitating detection, can be retrieved from it.
Typical systems of the above type operate with the so-called "Manchester code" and its different implementation, where the digital information may be formed so that there is always a change in polarity, irrespective of whether the information is a logical One or a logical Zero. This change in polarity is detected and utilized for correcting the frequency in a phase locked loop (PLL) oscillator, for example. In modern transmission systems of the so-called "burst" type, information is sent, for example, in one direction for 1/3 of the time and in the other for a further 1/3, while the remaining 1/3 must be unutilized to compensate for the propagation times in the line.
The problems in systems utilizing the Manchester code include that an error is more or less assumed in the related self-correcting apparatus, an error voltage thus being generated with the intention of correcting the frequency, and that when the error is corrected the error voltage disappears and a new error occurs. The system thus affords a natural tendency for the frequency to oscillate about the correct one, and is therefore seldom exactly right. The digital information is furthermore affected by the characteristics of the line and the sequence of the logical Ones or Zeros, particularly the latter if the information is not continuous. These conditions result in that the detections will not be exact in time, but vary somewhat so that jitter occurs. This jitter must be kept within given limits if detection errors are not to occur.
The problem in utilizing the burst system is that apart from the synchronizing information being absent for 2/3 of the time, as previously mentioned, the start of the information packet must be determined with great accuracy. The packet is thus characterized by a time period having no signal, after which the signals occur. Irrespective of whether the subsequent signals are DC-symmetric (alternating +Ones and -Ones) the first signals will be of different magnitudes and with a displacement in the zero passages. The first pulse in a pulse train starts with the line in the rest condition of 0 volts. On the other hand, the second pulse is affected by the residual voltage from the previous pulse. This means that each individual pulse is thus dependent on its antecedents.